Azithromycin monohydrate isopropanol clathrate and methods for the manufacture thereof

ABSTRACT

A novel form of azithromycin and processes for preparation of pure azithromycin monohydrate isopropanol clathrate (3 molecules of isopropanol for every 10 molecules of azithromycin monohydrate) has been obtained. Preparation of the novel form of azithromycin comprises the steps of dissolving azithromycin in isopropanol, followed by the slow addition of water to the organic solution.

FIELD OF THE INVENTION

This invention relates to a new form of azithromycin, namelyazithromycin monohydrate isopropanol clathrate, which has improvedproperties over amorphous azithromycin, azithromycin monohydrate andazithromycin dihydrate. This invention also relates processes for themanufacture of azithromycin monohydrate isopropanol clathrate.

BACKGROUND OF THE INVENTION

Azithromycin, 9-Deoxo-9a-aza-9a-methyl-9a-homoerythromycin A, is asemi-synthetic macrolide antibiotic which can be classified as a memberof the second-generation erythromycin antibacterial agent. Azithromycinhas the following structure (I):

The spectrum of azithromycin's antibacterial activity has been reportedby Aronoff, et al (J. Antimicrob. Chemother, 1987, 19, 275). Its mode ofaction has been reviewed by Retsema, et al (Antimicrob. Ag. Chemother,1987, 31, 1939)n, and its pharmacology has been reviewed by a number ofinvestigators (J. Antimicrob. Chemother., 1993, 31, Suppl. E, 1-198).

Three forms of Azithromycin are known. Anhydrous azithromycin isreported as an amorphous crude product (foam) in Canadian Patent 1 191843 (example 1). It is obtained by evaporating the final solvent (e.g.chloroform) used in the process of preparation of azithromycin. It isnot a crystalline product and therefore can not be made in pure form perse in commercial scale. In laboratory scale, it can be obtained in pureform by chromatography of the crude final product or by dissolving purecrystalline azithromycin mono- or dihydrate in an organic solvent andevaporating the said solvent to obtain amorphous anhydrous azithromycin.

Canadian patents 1,202,620, 1,202,619, 1,202,963 and 1,314,876 teach theprocess of making azithromycin monohydrate but do not claim theresulting product. Furthermore, these patents do not provide adescription of the drying process (temperature or pressure). Canadianpatents 1,191,843 and 1,202,963 claim azithromycin monohydrate as a newform of azithromycin. The theoretical percentage of water inazithromycin monohydrate is 2.3%. However, Canadian Patent 1,314,876reports a value of 3.92%, and a value of 3.2% is reported in Canadianpatent 1,314,876. No reference to the percentage of water is made in theother above-mentioned Canadian patents. Azithromycin monohydrate isknown to be hygroscopic (see for example European Patent 298 650 B1).This is an undesirable property since it complicates formulation ofazithromycin drug product and can adversely effect its stability on longterm storage.

Canadian patent 1,314,876 claims azithromycin dihydrate and, in contrastto azithromycin monohydrate, a full description of the drying processused for obtaining the product is provided. Low boiling, solvents(tetrahydrofuran and hexane) are used with 3-4 equivalent moles of waterto obtain the crystalline product, which is dried under vacuum at lowtemperatures (20-10° C.). The use of low boiling solvents forcrystallisation and low temperatures for vacuum drying of the productare prescribed presumably to control the desirable amount of water thatmust be evaporated to afford azithromycin dihydrate. Excess loss ofwater, caused by higher temperature vacuum drying, could result in theformation of azithromycin monohydrate. In contrast to anhydrousazithromycin and azithromycin monohydrate, azithromycin dihydrate hasdesirable properties for formulation. It is crystalline and cantherefore be obtained in pure form in commercial scale. It is nothygroscopic and therefore does not pose a problem during formulation oradversely effect the stability of the resulting drug product.

It is clear that anhydrous and monohydrate forms of azithromycin are notsuitable for formulation. The processes referred to in Canadian Patent 1314 876 for the preparation of azithromycin dihydrate, while producing anon-hygroscopic form of azithromycin, have a number of disadvantages:

-   -   1. Water immiscibility of the organic solvent mixture        (tetrahydrofuran plus hexane) can cause problems in obtaining        pure material since crystallisation processes are known to        afford pure material when the anti-solvent is miscible with the        solvent used to dissolve the crude product.    -   2. The drying process must be very carefully controlled since an        increase in temperature will cause the transformation of the        non-hygroscopic dihydrate to the hygroscopic monohydrate.    -   3. The use of low boiling point solvents is complicated by their        toxicity and possibility of formation of explosive peroxide        during solvent recovery.

It has now been surprisingly found that slow addition of water to anisopropanol solution of azithromycin results in the formation of a newform of azithromycin, namely azithromycin monohydrate isopropanolclathrate of formula II:

The physical properties of this product and the processes used for itspreparation have a number of major advantages over the existingazithromycin product forms and the procedures used for theirpreparation.

First, azithromycin monohydrate isopropanol clathrate is crystallineand, in contrast to anhydrous azithromycin, may be obtained in pureform.

Second, azithromycin monohydrate isopropanol clathrate is nothygroscopic and, in contrast to anhydrous azithromycin and azithromycinmonohydrate, may be used in formulations of the drug product as tabletsor capsules with excellent stability profiles.

Third, azithromycin monohydrate isopropanol clathrate is, in contrast toazithromycin dihydrate, obtained conveniently and reproducibly bycrystallisation from isopropanol water.

Fourth, in contrast to azithromycin dihydrate, azithromycin monohydrateisopropanol clathrate is obtained by crystallisation from inexpensivesolvents.

Fifth, in contrast to azithromycin dihydrate, azithromycin monohydrateisopropanol clathrate is prepared from environmentally safe solvents(hexane: Class 2; isopropanol and tetrahydrofuran: Class 3, see FederalRegister, Vol. 62, No. 247, 67381, Dec. 24, 1997).

Sixth, the experimental conditions are simple and applicable tolarge-scale production.

Seventh, the present processes are reproducible in a wide spectrum ofphysical conditions and consistently afford azithromycin monohydrateisopropanol clathrate with a constant ratio of azithromycin, water andisopropanol (vacuum drying at 1-10 mm Hg at 500 to 60° C. for 12 to 24hours).

Eighth, the product generated by the processes of the present inventionis highly pure.

Ninth, the processes taught in this invention afford high yields of theproduct within the range of 88% to 93% (first crop). The remainder ofthe product is conveniently recovered from the mother liquor byevaporation of isopropanol under reduced pressure.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, the invention relates to a compound of formula II:

In another aspect, the invention relates to a process for thepreparation of azithromycin monohydrate isopropanol clathrate whichcomprises the steps of:

-   -   (a) Dissolving azithromycin in isopropanol and slowly adding        water to the resulting solution;    -   (b) Filtering and washing the product with a mixture of        isopropanol water;    -   (c) Vacuum drying the product.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 is a powder X-Ray diffraction of anhydrous azithromycin.

FIG. 2 is a powder X-Ray diffraction of azithromycin monohydrate.

FIG. 3 is a powder X-Ray diffraction of azithromycin monohydrateisopropanol clathrate.

FIG. 4 is a powder X-Ray diffraction of azithromycin dihydrate.

FIG. 5 is a DSC of azithromycin monohydrate.

FIG. 6 is a DSC of azithromycin monohydrate isopropanol clathrate.

FIG. 7 is an IR spectrum of azithromycin monohydrate and azithromycinmonohydrate isopropanol clathrate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention describes a new form of azithromycin monohydrate,namely azithromycin monohydrate isopropanol clathrate and the processesfor the preparation of pure azithromycin monohydrate isopropanolclathrate.

Previously known forms of azithromycin (anhydrous, monohydrate, anddihydrate) may serve as the starting material in the present, all ofwhich are commercially available.

According to this invention, azithromycin monohydrate isopropanolclathrate contains three molecules of isopropanol for every tenmolecules of azithromycin monohydrate. The process comprises thedissolution of azithromycin in isopropanol to which water is addedslowly while stirring, resulting in the precipitation of crystallineazithromycin monohydrate isopropanol clathrate. The volume of solventused is such as to be sufficient to dissolve azithromycin. The additionof the water is carried out between 0° and 30° C. and preferably between15° C. to 25° C. The product is filtered and washed with a mixture ofwater-isopropanol and dried under vacuum (1-10 mm Hg) at 50° C. to 60°C. for 12-24 hours to obtain azithromycin monohydrate isopropanolclathrate in high yields. Extension of vacuum drying does not reduceeither the water content or the isopropanol content of azithromycinmonohydrate isopropanol clathrate.

Elemental analysis, 1HNMR, 13C NMR, and IR spectroscopy, massspectrometry, and powder x-ray diffraction and IR have identified theazithromycin monohydrate isopropanol clathrate produced according to theinvention. FIGS. 1 to 4 show the differences between powder x-raydiffraction of anhydrous azithromycin, azithromycin monohydrate.,azithromycin monohydrate isopropanol clathrate, and azithromycindihydrate. Comparison of FIG. 3 with FIGS. 1, 2 and 4 clearly shows thedifferences in the morphology of azithromycin monohydrate isopropanolclathrate with anhydrous azithromycin, azithromycin monohydrate andazithromycin dihydrate. These figures also indicate that azithromycinmonohydrate isopropanol clathrate is free of azithromycin dihydrate.

Differential Scanning Colorimetry (DSC) of azithromycin monohydrate(157.99° C.) and azithromycin monohydrate isopropanol clathrate (149.88°C.) are shown in FIGS. 5 and 6.

Near IR spectra of azithromycin monohydrate and azithromycin monohydrateisopropanol clathrate are shown in FIG. 7. The major difference is at6800 cm⁻¹ at which the clathrate shows a medium absorption.

The water content of azithromycin monohydrate isopropanol clathrate wasmeasured by the Karl-Fischer method and its isopropanol content wasdetermined by gas chromatography.

X-RAY DIFFRACTION Instrumental Parameters Instrument: Philips PW3710Based Diffractometer with APD Software Ver. 3.6 Sample preparationunground Radiation: CuKα₁(λ = 1.54056 Å) Scanning Mode: Step ScanningRange 4.0-40.0 (°2θ): Step Size (°2θ): 0.020 Measuring Time 1.20(sec/step): Holder type: Phillips Standard Operation 40 KV × 40 mA Power0.5° Divergence Slit: Receiving Slit: 0.2 mm Scattering Slit: 0.5°Relative Intensity Angle (°2θ) D-value (Å) % 4.985 17.712 0.2 5.60515.754 0.3 6.205 14.232 1.3 7.350 12.017 1.7 7.855 11.246 7.5 8.24010.721 0.4 8.830 10.006 0.3 9.400 9.401 4.1 9.790 9.027 100.0 10.2458.627 0.4 11.165 7.918 8.8 11.365 7.779 2.5 11.935 7.409 1.4 12.4957.078 4.3 13.955 6.341 2.2 14.250 6.210 1.2 14.645 6.044 2.6 14.8105.977 1.8 15.270 5.798 5.3 15.700 5.640 2.9 15.990 5.538 0.9 16.5955.338 1.1 17.040 5.199 2.1 17.450 5.078 1.5 18.035 4.915 0.5 18.3754.824 1.0 18.540 4.782 1.0 19.060 4.653 2.8 19.670 4.510 2.8 19.9954.437 1.7 20.425 4.345 2.7 20.885 4.250 1.1 21.030 4.221 0.8 21.7404.085 0.8 22.540 3.941 0.8 23.470 3.787 0.5 24.125 3.686 0.6 24.4753.634 0.7 24.705 3.601 0.7 25.245 3.525 0.6 25.510 3.489 0.9 26.1453.406 0.8 26.510 3.360 0.2 28.320 3.145 0.3 29.200 3.056 0.3 29.4103.035 0.3 29.825 2.993 0.2 30.170 2.960 0.2 32.750 2.732 0.4 33.5652.668 0.4 34.640 2.587 0.2 35.295 2.541 0.3 36.135 2.484 0.3 37.4902.397 0.2 39.710 2.268 0.2

The invention will be more fully understood by the following examples,which illustrate the present invention, but are not to be consideredlimiting to the scope of the invention.

EXAMPLE 1

Anhydrous azithromycin (1 kg ) is dissolved in isopropanol (2.8 kg) bywarming. The solution is stirred vigorously and water (4.35 kg) is addedslowly over a 1-hour period. The mixture is cooled to 20° C. and stirredfor an additional 6 hours at this temperature. The resulting product isfiltered and washed with a 40:60 mixture of isopropanol-water. The cakewas then dried vacuum (6 to 10 mm Hg) at 50° C. for 12 hours. Yield 0.88kg (88%).

EXAMPLE 2

Azithromycin monohydrate (1 kg) is dissolved in isopropanol (2.8 kg) bywarming. The solution is stirred vigorously and water (4.35 kg) is addedslowly over a 1-hour period. The mixture is cooled to 20° C. and stirredfor an additional 6 hours at this temperature. The resulting product isfiltered and washed with a 40:60 mixture of isopropanol-water. The cakewas then dried vacuum (6 to 10 mm Hg) at 50° C. for 12 hours. Yield 0.88kg (88%).

EXAMPLE 3

Azithromycin dihydrate (1 kg) is dissolved in isopropanol (2.8 kg) bywarming. The solution is stirred vigorously and water (4.35 kg) is addedslowly over a 1-hour period. The mixture is cooled to 20° C. and stirredfor an additional 6 hours at this temperature. The resulting product isfiltered and washed with a 40:60 mixture of isopropanol-water. The cakewas then dried vacuum (6 to 10 mm Hg) at 50° C. for 12 hours. Yield 0.88kg (88%).

1. A compound of formula II:


2. A process for the preparation of azithromycin monohydrate isopropanolclathrate which comprises the steps of: (a) dissolving azithromycin inisopropanol and slowly adding water to the resulting solution so that aprecipitate of crystalline azithromycin monohydrate isopropanolclathrate is formed; (b) filtering and washing the product resultingfrom step (a) with a mixture of isopropanol and water; and (c) vacuumdrying the product resulting from step (b).
 3. The process of claim 2wherein the dissolution of crystalline azithromycin is carved out in avolume of solvent only sufficient to dissolve the azithromycin.
 4. Theprocess of claim 2 wherein water is added over a period of one hour. 5.The process of claim 2 wherein the addition of water to the resultingsolution is carried out between 0° C. to 30° C.
 6. The process of claim5 wherein the addition of water is carried between 15° C. to 25° C. 7.The process of claim 2 wherein vacuum drying is carried out at atemperature of 50° C. to 60° C.
 8. The process of claim 2 wherein thevacuum drying is carried out under 6 to 10 mm Hg.
 9. A process for thepreparation of azithromycin monohydrate isopropanol clathratecharacterised by the following x-ray powder diffraction patternexpressed in terms of “D” spacings and Relative Intensity: RelativeIntensity Angle (°2θ) D-value (Å) % 4.985 17.712 0.2 5.605 15.754 0.36.205 14.232 1.3 7.350 12.017 1.7 7.835 11.246 7.5 8.247 10.721 0.48.830 10.006 0.3 9.400 9.401 4.1 9.790 9.027 100.0 10.245 6.627 0.411.165 7.918 8.8 11.365 7.779 2.5 11.935 7.409 1.4 12.495 7.078 4.313.055 6.341 2.2 14.250 6.210 1.2 14.645 6.044 2.6 14.810 3.977 1.815.270 5.798 3.3 15.700 5.640 2.9 15.990 5.338 0.9 16.595 5.338 1.117.040 5.199 2.1 17.450 5.078 1.5 18.035 4.915 0.5 18.375 4.824 1.018.540 4.782 1.0 19.060 4.613 2.8 19.670 4.510 2.8 19.995 4.437 1.720.425 4.315 2.7 20.885 4.150 1.1 21.030 4.221 0.8 21.740 4.085 0.822.540 3.941 0.8 23.470 3.787 0.5 24.125 3.680 0.6 24.475 3.634 0.724.705 3.601 0.7 25.245 3.525 0.6 25.510 3.489 0.9 26.145 1.406 0.826.510 3.360 0.2 28.320 3.145 0.3 29.200 3.056 0.3 29.410 3.035 0.329.825 2.993 0.2 32.750 2.732 0.4 33.565 2.668 0.4 34.640 2.587 0.235.295 2.541 0.3 36.133 2.484 0.3 37.490 2.397 0.2 39.710 2.268 0.2

which comprises the steps of: (a) dissolving azithromycin in isopropanoland slowly adding water to the resulting solution so that a precipitateof crystalline azithromycin monohydrate isopropanol clathrate is formed;(b) filtering and washing the product resulting from step (a) with amixture of isopropanol and water; and (c) vacuum drying the productresulting from step (b).
 10. The compound of claim 1 which is pure. 11.The compound of claim 10 which is free of azithromycin dihydrate. 12.The compound of claim 1 formulated as a drug product.
 13. The compoundof claim 12 in tablet form.
 14. The compound of claim 12 in capsuleform.
 15. Crystalline azithromycin monohydrate isopropanol clathrate.16. The compound of claim 15 which is pure.
 17. The compound of claim 16which is free of azithromycin dihydrate.
 18. The compound of claim 15formulated as a drug product.
 19. The compound of claim 18 in tabletform.
 20. The compound of claim 18 in capsule form.
 21. Azithromycinmonohydrate isopropanol clathrate characterized by the x-ray diffractionpattern as illustrated in FIG.
 3. 22. The compound of claim 21 which ispure.
 23. The compound of claim 22 which is free of azithromycindihydrate.
 24. The compound of claim 21 formulated as a drug product.25. The compound of claim 24 in tablet form.
 26. The compound of claim24 in capsule form.
 27. The compound of claim 21 wherein the DSC thereofis illustrated in FIG.
 6. 28. The compound of claim 21 furthercharacterized by the IR spectrum thereof as illustrated in FIG. 7 29.The compound of claim 28 further characterized by the DSC thereofillustrated in FIG.
 6. 30. Azithromycin monohydrate isopropanolclathrate characterized by the IR spectrum thereof as illustrated inFIG.
 7. 31. The compound of claim 30 wherein the DSC thereof isillustrated in FIG.
 6. 32. The compound of claim 30 which is pure. 33.The compound of claim 32 which is free of azithromycin dihydrate. 34.The compound of claim 30 formulated as a drug product.
 35. The compoundof claim 34 in tablet form.
 36. The compound of claim 34 in capsuleform.
 37. Azithromycin monohydrate isopropanol clathrate characterizedby the following x-ray powder diffraction pattern expressed in terms of“D” spacings and Relative Intensity: Relative Intensity Angle (°2θ)D-value (Å) % 4.986 17.712 0.2 5.605 15.754 0.3 6.205 14.232 1.3 7.35012.017 1.7 7.855 11.246 7.5 8.240 10.721 0.4 8.830 10.006 0.3 9.4009.401 4.1 9.790 9.017 100.0 10.245 4.827 0.4 11.165 7.918 8.8 11.3687.779 2.5 11.938 7.409 1.4 12.485 7.078 4.3 13.935 6.941 2.2 14.2506.210 2.3 14.545 6.044 2.6 14.810 1.977 1.8 15.270 5.640 1.9 16.5951.378 1.1 17.040 5.199 2.1 17.450 5.078 2.5 18.375 4.284 1.0 18.3404.782 1.0 19.080 4.510 2.8 19.995 4.437 1.9 20.425 4.349 2.7 20.8894.150 1.1 21.050 4.321 0.8 21.740 4.085 0.8 22.540 3.941 0.8 23.4703.787 0.5 24.475 1.634 0.7 24.705 3.801 0.7 25.245 3.525 0.4 25.5101.489 0.9 25.149 3.406 0.8 26.510 3.360 0.3 28.320 3.145 0.3 29.2003.096 0.3 29.410 3.039 0.3 29.829 2.993 0.2 30.170 2.960 0.2 32.1502.732 0.4 33.365 2.668 0.4 34.640 2.587 0.3 35.291 2.541 0.3 36.1352.484 0.3 37.490 2.397 0.2 39.710 2.268 0.2.


38. The compound of claim 37 which is pure.
 39. The compound of claim 38which is free of azithromycin dihydrate.
 40. The compound of claim 37formulated as a drug product.
 41. The compound of claim 40 in tabletform.
 42. The drug product of claim 40 in capsule form.